This boss is hard because he is surrounded by enemies and even more spawn once inside, including the boss Falaharn Mistwarden. An easier way to combat him is to go around to the side where Falaharn spawns and continually pull enemies out and dispose of them. Pretty soon, Sunreach's squad will be the only remaining group of enemies; you can then proceed to pull his group out and onto the hill near Durheim Archives and focus all your strength onto Sunreach himself.